Laser Carrots

Guess what you can do with a plate of lovely vegetables? And it's got nothing to
do with sex or slimming. Put down that fork. Show your coleslaw a little more
respect. For Sliced carrots could help to unlock some of the strangest secrets
of the Universe!

The tangled history of the Universe could be unravelled by the light from
a glowing carrot. Now there's food for thought.

If you don't believe it, just take a peek into Hiroshi Taniguchi's lab at
Iwate University in Japan. He has turned to thick chunks of carrot and potato
to unravel some pretty indigestible physics. It may sound crazy that a fresh
salad could make anything more than a low-calorie lunch, but these aren't
any old vegetables--they are vegetable lasers.

Taniguchi's recipe is simple. Take your freshly sliced vegetables and dunk
them in his special sauce--actually a fluorescent dye--blast them with a
laser beam of just the right wavelength and watch the slices glow.

According to Taniguchi's research, potatoes work well, but so do carrots,
green peppers and pumpkins. He has even used grains of rice. But any vegetable,
freshly prepared, should be well-suited to life as a guiding light. They
shine so brightly, says Taniguchi, that you don't even need to darken the
lab to pick them out. And although no one is exactly sure how these vegetable
lasers work, one day they could provide the clues that astronomers need to
decode mysterious broadcasts from watery clouds in far-off galaxies.

When you think of a laser, sliced vegetables are probably not the first thing
that comes to mind. Lasers usually contain less palatable things, a chunk
of ruby or a glass cylinder of argon gas, for instance, in a cavity between
two highly polished and precisely aligned mirrors. Trigger the atoms in the
cavity with light from a flashlamp and they emit more of the same. As the
photons bounce back and forth between the mirrors, the light is amplified
until, in a fraction of a second, the cavity floods with laser light.

Taniguchi's vegetable lasers are far less sophisticated. Instead of ruby
or argon, they rely on molecules of his dye that cling to the vegetables
after dunking. The dye absorbs light and re-emits it as fluorescence. Rather
than mirrors, the cavity is formed by the walls of the tiny, randomly-oriented
plant cells that make up the vegetable tissue. And instead of a flashlamp,
the power for Taniguchi's vegetables comes from another laser beam.

When his vegetables give off their ghostly glow, Taniguchi believes that
the fluorescing photons inside them become "coherent". In other words,
they stop behaving as individuals and take on the characteristics of laser
light in which the wavelength and phase of every photon is identical. He
thinks he has created something called a multiple light scattering laser--or
random laser.

Physicists have argued for decades that it should be possible to create this
new kind of laser using photons that follow random paths. Shine light into
something that scatters light efficiently, so the theory goes, and if they
are deflected often enough in random directions, the chances are that some
of them will follow repetitive, circular paths. If they are amplified as
they ricochet around, and if the wavelength of light matches the lengths
of these random loops, the photons will lock their wavelengths and phases
together. In theory, the material should switch on like a light bulb, lit
up by laser beams.

This year, physicist Hui Cao and her colleagues at Northwestern University
in Illinois turned this theory into practice. In March, they published a
report in Physical Review Letters (vol 82, p 2278) that showed they had turned
a powder into a random laser. They laid an ultrafine powdered mixture of
zinc oxide and gallium nitride onto a piece of glass and fired rapid bursts
of bright blue laser light at it. The zinc oxide is fluorescent so it acted
like the dye in Taniguchi's vegetables. The tiny particles of zinc oxide
and gallium nitride--each a mere 100 nanometres across--are very efficient
scatterers, so the photons changed direction after travelling only very short
distances. "Now there's a chance that the light is going to come back on
itself," says Cao. Her powder forms billions of minuscule "ring cavities"
that amplify the light, just like the cavity in an ordinary laser.

With this in mind, Taniguchi isn't too surprised that it's possible to build
lasers from little more than sliced vegetables. "We knew that almost all
vegetables have the continuously disordered fine structures required for
random lasers," he says.

And back in his lab, Taniguchi has discovered something else: you don't even
need to throw away the vegetable waste left over from preparing your lasers.
He extracts a pigment from radish leaves and uses it to create a fluorescent
dye. When he injects the dye into an emulsion of biological fats that scatters
light--something exactly like mayonnaise, for example--it too behaves as
a laser. The meal is complete: inject the mayonnaise with dye, turn on the
laser and your coleslaw or potato salad becomes a truly light lunch.

If you're worried by the prospect of being blinded by the beam from a carrot
laser, fear not. Random lasers will probably never make high intensity sources:
the light is entirely unfocussed and comes out in all directions--which explains
why Taniguchi's veg have such a ghostly glow. But Cao is looking at the random
laser as a possible new display technology. "Although the light can go in
all different directions, it's got pretty high efficiency," she says. "It's
like a light-emitting diode."

Light-emitting coleslaw or a potato shining like a full moon may not be the
most practical of devices, but Taniguchi believes they will be useful
nonetheless. There's loads these lasers can teach us about multiple light
scattering, he says. And luminous comestibles may have other uses: they could
offer physicists the tool they need to develop new techniques for identifying
molecules or particles in highly scattering environments.

Physicists already use light scattering to look inside things such as tissue,
blood or suspensions of fats. But these measurements are tricky: try to beam
light into something like milk and most is scattered straight back out before
it can reach whatever it is you're looking for. If you want to probe more
than a few centimetres beneath the surface, you need an incredibly bright
light, and a large helping of luck.

So why not inject your sample with micrometre-diameter spheres containing
a fluorescent dye. Now you can use a laser to excite the dye and create a
random laser inside your sample. With the extra light this creates, scattering
studies should be far easier. Perhaps one day this approach could make it
simpler to identify rogue bugs inside vats of stout, or cancerous cells lurking
deep inside living tissue.

But those most likely to benefit from a plateful of glowing vegetables will
be astronomers. For laser salads have cosmic potential for shedding light
on the workings of strange lasers in distant galaxies.

Deep in the heart of galaxy NGC4258, for instance, is a cloud of water vapour
which blasts out radiation with astonishing power. Researchers believe that,
somehow, energy is being pumped into the water molecules, stimulating them
and turning them into a gigantic microwave laser or "maser". The power for
this maser, astronomers believe, comes from energetic X-rays beamed out of
a supermassive black hole that lurks near the centre of the galaxy.

Understanding the way the maser works could tell us useful things about the
black hole, such as its rate of growth, which could in turn tell us something
about the first moments of the Universe. And the Universe is peppered with
billions of similar masers. Their radiation could also provide clues to the
way stars form and die--information that cosmologists would find invaluable.

The problem, says Harvard University astronomer James Moran, is that masers
in galaxies such as NGC4258 are constantly changing, which makes them difficult
to understand. The clouds of vapour are moving across the galaxy at thousands
of kilometres per hour, and their shapes keep changing. With the pattern
of maser emission altering constantly, understanding how they work is difficult.
"The details aren't understood very well because masers are finicky," Moran
says.

Enter the humble vegetable. The random orientation of plant cells that turns
a salad into a laser may mimic the random arrangement of water molecules
in a shimmering maser cloud. "It could be similar," Moran admits.

A terrestrial random laser might be just the tool for bringing the subject
down to Earth. Cao believes her study of random lasers--based on scattering
mechanisms similar to galaxy masers--could also help. And her piles of
semiconductor powder seem to be lasing in exactly the same way as the sliced
vegetables in Taniguchi's laboratory. So it's not inconceivable the tangled
history of the Universe could be unravelled by the light from a glowing carrot.
Now there's food for thought.

Using Nobel laureate C.V. Raman’s discovery, team has created a laser using a
carrot, which is important because it answers the call for eco-friendly devices.

Bengaluru: In a global first, researchers at IIT-Madras have created a
bio-friendly laser out of a carrot, using a process known as Raman Random
Lasing, discovered by Nobel laureate C.V. Raman.

The research was undertaken by a team comprising PhD research scholar Venkata
Siva Gummaluri, assistant professor Sivarama Krishnan and professor C. Vijayan
of IIT-Madras’ Physics Department.

As lasers find more and more applications in daily life, the invention is
important because it answers the increasing calls to make devices eco-friendly.
The team’s results were published in a paper in the Optics Letters journal from
the Optical Society of America.

How lasers work in a carrot

Light Amplification by Stimulated Emission of Radiation, or laser, is a
highly collated beam of light that can be focused into a microscopic point. The
intense energy makes lasers useful for cutting and cauterising.

They are used widely in the medical field for precision, and also find
widespread use in industries such as textiles, electronics and data storage,
nuclear fusions, communications, security systems and scanners, defence, and
more.

Lasers work on the principle of amplification of light emitted by stimulating
electromagnetic sources. A ‘gain medium’ in the design is a material that
amplifies resulting signals, resulting in the increase in optical power. The IIT-Madras
team utilised carrots as the gain medium, taking advantage of naturally
occurring fibrous cellulose, and carotene, a photosynthetic pigment that is the
precursor to vitamin A.

“We were excited to see lasing in fresh carrots, due to the carotene and
cellulose found in them,” said Gummaluri, lead author of the new paper.

In a random laser — also called a stochastic laser — a scattering process is
used to bounce light randomly between particles to trap light long enough for
the medium to amplify it. In conventional lasers, this is achieved by mirrors
that reflect the light back and forth, but a turbid medium has been demonstrated
to achieve this with much more intensity.

The new carrot laser also utilises CW laser pumping, where energy is provided
constantly without breaks to amplify the laser, which then produces a continuous
stream of output instead of pulses. Such lasers have been built with bio
materials in the past, but most such gain media (like dyes) are known to be
toxic, requiring excessive care when handling. They have also not been
bio-degradable.

“Organic bio-pigments like carotenoids found in carrots and porphyrins found
in chlorophyll are interesting optically-active media because of their visible
light absorption properties,” explained Krishnan. “There is now a move towards
development of green, sustainable materials for various applications, including
in photonics. The need for green photonic technologies in obvious in the current
times where sustainability, bio-compatibility and bio-degradability are of
paramount importance.”

The new laser, apart from being eco-friendly and biodegradable, is also
expected to scale in a more cost-effective manner.